Elemental Composition (C, N, P) and Cell Volume of Exponentially Growing and Nutrient-Limited Bacterioplankton

Marine bacterioplankton were isolated and grown in batch cultures until their growth became limited by organic carbon (C), nitrogen (N), or phosphorus (P). Samples were taken from the cultures at both the exponential and stationary phases. The elemental composition of individual bacterial cells was analyzed by X-ray microanalysis with an electron microscope. The cell size was also measured. The elemental content was highest in exponentially growing cells (149 ± 8 fg of C cell⁻¹, 35 ± 2 fg of N cell⁻¹, and 12 ± 1 fg of P cell⁻¹; average of all isolates ± standard error). The lowest C content was found in C-limited cells (39 ± 3 fg of C cell⁻¹), the lowest N content in C- and P-limited cells (12 ± 1 and 12 ± 2 fg of N cell⁻¹, respectively), and the lowest P content in P-limited cells (2.3 ± 0.6 fg of P cell⁻¹). The atomic C:N ratios varied among treatments between 3.8 ± 0.1 and 9.5 ± 1.0 (average ± standard error), the C:P ratios between 35 ± 2 and 178 ± 28, and the N:P ratios between 6.7 ± 0.3 and 18 ± 3. The carbon-volume ratios showed large variation among isolates due to different types of nutrient limitation (from 51± 4 to 241 ± 38 fg of C μm⁻¹; average of individual isolates and treatments ± standard error). The results show that different growth conditions and differences in the bacterial community may explain some of the variability of previously reported elemental and carbon-volume ratios.